Method and apparatus for controlling soft buffer for TDD-FDD carrier aggregation

ABSTRACT

Provided is a method and apparatus for controlling soft buffer for TDD-FDD carrier aggregation. The method includes: establishing a Radio Resource Control (RRC) connection with a base station through a first serving cell, the first serving cell supporting a Time Division Duplex (TDD) mode; receiving an RRC message from the base station through the first serving cell, the RRC message comprising carrier aggregation (CA) configuration information, the CA configuration information comprising information of a second serving cell supporting a Frequency Division Duplex (FDD) mode; determining a maximum number of DL HARQ processes for the second serving cell, the maximum number of DL HARQ processes for the second serving cell being differently determined according to a DL reference timing; and storing soft channel bits for a received transport block (TB) based on the determined maximum number of DL HARQ processes for the second serving cell.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from and the benefit of Korean PatentApplication No. 10-2013-0162327, filed on Dec. 24, 2013, which is herebyincorporated by reference in its entirety.

BACKGROUND

1. Field

The present disclosure relates to a wireless communication, and moreparticularly, to a method and apparatus for controlling soft buffer forTDD-FDD carrier aggregation.

2. Discussion of the Background

The wireless communication system may support Frequency Division Duplex(FDD) and Time Division Duplex (TDD). In the FDD, a carrier used for anuplink (UL) transmission and a carrier used for a downlink (DL)transmission exist, respectively, and both the UL transmission and theDL transmission are simultaneously executed in a cell. In the TDD, a ULtransmission and a DL transmission are distinguished from each other,based on a time, in a single cell. In the TDD, an identical carrier maybe used for a UL transmission and a DL transmission Thus, a base stationand a UE repeatedly execute conversions between a transmission mode anda reception mode. The TDD includes a special subframe so as to provide aguard time for converting a mode between transmission and reception. Thespecial subframe may include a Downlink Pilot Time Slot (DwPTS), a GuardPeriod (GP), and an Uplink Pilot Time Slot (UpPTS), as illustrated inthe drawings, e.g., FIG. 3. For the TDD, through various UL-DLconfigurations, resources may be asymmetrically allocated for UL and DLtransmissions.

For an effective, reliable communication, a Hybrid Automatic RepeatRequest (HARQ) process may be used. Unlike an Automatic Repeat Request(ARQ) process, a Forward Error Correcting Code (FEC) may be used for theHARQ process. For example, if a receiver correctly decodes a data signal(or a packet), the receiver may feedback an acknowledgement (ACK) so asto inform the transmitter that the data signal was correctly decoded. Ifthe receiver fails to correctly decode a data signal, the receiver mayfeedback a negative acknowledgement to the transmitter to inform thetransmitter of the decoding failure of the data signal. A User Equipment(UE) may store a part or entirety of data corresponding to the datasignal in a soft buffer of the UE. The UE receives a packetretransmitted from the transmitter, and combines the stored data and theretransmitted data, so as to increase a probability of success ofdecoding. The UE continuously executes the HARQ process until the packetis correctly decoded or until a predetermined maximum number ofretransmissions are executed. Therefore, a space of the soft bufferneeds to be reserved for the HARQ process related to a packet that theUE fails to correctly decode. When the soft buffer is fully utilized, orthe space is insufficient, the UE may not execute the HARQ process.

Currently, frequency resources are scarce and various technologies areused in a part of the broad frequency bands. For this reason, to satisfya higher data transmission rate requirement, as a scheme for securing abroadband bandwidth, each scattered band is designed to satisfy basicrequirements for operating an independent system and a CarrierAggregation (CA) has been employed, which binds up a plurality of bandsas a single system. A band or a carrier that may independently operatemay be defined as a Component Carrier (CC). Further, a TDD-FDD CAsupports a CA of an FDD carrier and a TDD carrier. When the TDD-FDD CAsystem is utilized, it may be required to execute the HARQ processesassociated with a plurality of Component Carriers. More specifically,the HARQ processes associated with the TDD and FDD carriers may need tobe stored in the soft buffer of the UE. Therefore, when the TDD-FDD CAis configured for the UE, there is a desire for a method of processingor controlling the soft buffer of the UE for the HARQ processesassociated with a plurality of CCs.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form any part of theprior art nor what the prior art may suggest to a person of ordinaryskill in the art.

SUMMARY

Exemplary embodiments of the present invention provide a method andapparatus for controlling soft buffer for TDD-FDD carrier aggregation.

An exemplary embodiment of the present invention provides a method ofperforming a Hybrid Automatic Repeat reQuest (HARQ) operation by a userequipment (UE), the method including: establishing a Radio ResourceControl (RRC) connection with a base station through a first servingcell, the first serving cell supporting a Time Division Duplex (TDD)mode, and a TDD uplink(UL)-downlink(DL) configuration of the firstserving cell being one of 1, 2, 3, 4, and 5; receiving an RRC messagefrom the base station through the first serving cell, the RRC messageincluding carrier aggregation (CA) configuration information, the CAconfiguration information including information of a second serving cellsupporting a Frequency Division Duplex (FDD) mode, and the first servingcell and the second serving cell being aggregated by a TDD-FDD CAscheme; determining a maximum number of DL HARQ processes for the secondserving cell, the maximum number of DL HARQ processes for the secondserving cell being differently determined according to a DL referencetiming; and storing soft channel bits for a received transport block(TB) based on the determined maximum number of DL HARQ processes for thesecond serving cell.

An exemplary embodiment of the present invention provides a method ofperforming a Hybrid Automatic Repeat reQuest (HARQ) operation by a userequipment (UE), the method including: establishing a Radio ResourceControl (RRC) connection with a base station through a first servingcell, the first serving cell supporting a Time Division Duplex (TDD)mode, and a TDD uplink(UL)-downlink(DL) configuration of the firstserving cell being one of 1, 2, 3, 4, and 5; receiving an RRC messagefrom the base station through the first serving cell, the RRC messageincluding carrier aggregation (CA) configuration information, the CAconfiguration information including information of a second serving cellsupporting a Frequency Division Duplex (FDD) mode, and the first servingcell and the second serving cell being aggregated by a TDD-FDD CAscheme; determining a maximum number of DL HARQ processes for the secondserving cell according to the TDD UL-DL configuration of the firstserving cell, the maximum number of DL HARQ processes for the secondserving cell being greater than 8; and storing soft channel bits for areceived transport block (TB) based on the determined maximum number ofDL HARQ processes for the second serving cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a wireless communication systemaccording to an exemplary embodiment of the present invention.

FIG. 2 illustrates an example of a protocol structure for supporting amulti-carrier system according to an exemplary embodiment of the presentinvention.

FIG. 3 illustrates an example of a radio frame structure according to anexemplary embodiment of the present invention. This includes an FDDradio frame structure and a TDD radio frame structure.

FIG. 4 illustrates a case of an inter-band CA of serving cells havingdifferent TDD UL-DL configurations.

FIG. 5 illustrates an example of an FDD-TDD CA scheme according to anexemplary embodiment of the present invention.

FIG. 6 illustrates examples of capabilities of a UE for a TDD-FDD CAaccording to an exemplary embodiment of the present invention.

FIG. 7 illustrates examples of the maximum number of DL HARQ processeswhen a TDD-FDD CA is configured for a UE according to an exemplaryembodiment of the present invention.

FIG. 8 illustrates a comparison of a performance of a UE with respect toa single code block, based on an M_(DL) _(_) _(HARQ) value according toan embodiment of an exemplary embodiment of the present invention.

FIG. 9 illustrates an example of a soft buffer partitioning methodaccording to Method 1 and Method 2 and an example of a soft bufferpartitioning method according to Method 3.

FIG. 10 illustrates an example of soft buffer allocation according toMethod 3-1.

FIG. 11 illustrates an example of soft buffer allocation according toMethod 3-2.

FIG. 12 illustrates an example of soft buffer allocation according toMethod 4.

FIG. 13 illustrates examples of the maximum number of DL HARQ processesaccording to Case 1 and Case 2.

FIG. 14 illustrates an example of soft buffer allocation according toMethod 5-1.

FIG. 15 illustrates an example of soft buffer allocation according toMethod 5-2.

FIG. 16 illustrates an example of soft buffer allocation according toMethod 6.

FIG. 17 is a flowchart illustrating a soft buffer controlling method ofa UE according to an exemplary embodiment of the present invention.

FIG. 18 is a block diagram illustrating a UE according to an exemplaryembodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Exemplary embodiments of the present invention will be described morefully hereinafter with reference to the accompanying drawings, in whichexemplary embodiments of the invention are shown. Throughout thedrawings and the detailed description, unless otherwise described, thesame drawing reference numerals are understood to refer to the sameelements, features, and structures. In describing the exemplaryembodiments, detailed description on known configurations or functionsmay be omitted for clarity and conciseness.

Further, the terms, such as first, second, A, B, (a), (b), and the likemay be used herein to describe elements in the description herein. Theterms are used to distinguish one element from another element. Thus,the terms do not limit the element, an arrangement order, a sequence orthe like. It will be understood that when an element is referred to asbeing “on”, “connected to” or “coupled to” another element, it can bedirectly on, connected or coupled to the other element or interveningelements may be present.

Further, the description herein is related to a wireless communicationnetwork, and an operation performed in a wireless communication networkmay be performed in a process of controlling a network and transmittingdata by a system that controls a wireless network (e.g., a base station)or may be performed in a user equipment connected to the wirelesscommunication network.

FIG. 1 is a diagram illustrating a wireless communication systemaccording to an exemplary embodiment of the present invention.

According to FIG. 1, a wireless communication system 10 is widelydeployed in order to provide diverse telecommunication services, such asvoice and packet data. A wireless communication system includes at leastone base station 11 (BS). Each BS 11 provides telecommunication serviceto certain cells 15 a, 15 b, and 15 c. A cell may again be divided intomultiple sectors.

User equipment 12 (mobile station, MS) may be located at a certainlocation or mobile, and may also be referred to as different terms,including UE (user equipment), MT (mobile terminal), UT (user terminal),SS (subscriber station), wireless device, PDA (personal digitalassistant), wireless modem, and handheld device. A base station 11 mayalso be referred to as eNB (evolved-NodeB), BTS (Base TransceiverSystem), Access Point, femto base station, Home nodeB, and relay. A cellinclusively refers to various coverage areas, such as mega cell, macrocell, micro cell, pico cell, and femto cell.

Hereinafter, the term downlink refers to communication from a basestation 11 to a UE 12, and the term uplink refers to communication froma UE 12 to a base station 11. For downlink, a transmitter may be part ofa base station 11, and a receiver may be part of a UE 12. For uplink, atransmitter may be part of a UE 12 and a receiver may be part of a basestation 11. There is no limitation in the multiple access method appliedto a wireless communication system. Diverse methods can be used,including CDMA (Code Division Multiple Access), TDMA (Time DivisionMultiple Access), FDMA (Frequency Division Multiple Access), OFDMA(Orthogonal Frequency Division Multiple Access), SC-FDMA (SingleCarrier-FDMA), OFDM-FDMA, OFDM-TDMA, OFDM-CDMA. Uplink transmission anddownlink transmission can use either TDD (Time Division Duplex), whichuses different time locations for transmissions, or FDD (FrequencyDivision Duplex), which uses different frequencies for transmissions.

Carrier Aggregation (CA), which is also referred to as spectrumaggregation or bandwidth aggregation, supports multiple carriers. Eachindividual unit carrier, which is aggregated by carrier aggregation, isreferred to as Component Carrier (CC). Each component carrier is definedby bandwidth and center frequency. CA is introduced to supportincreasing throughput, to prevent cost increase due to the introductionof the wideband radio frequency and to ensure the compatibility with theexisting system. For example, if five component carriers are allocatedas granularity that has a carrier unit with 20 MHz bandwidth, it cansupport 100 MHz bandwidth at maximum.

CA may be divided as contiguous carrier aggregation, which is made amongcontinuous CCs, and non-contiguous carrier aggregation, which is madeamong non-continuous CCs. The number of carriers aggregated betweenuplink and downlink may be configured differently. It is referred to assymmetric aggregation when there are equal number of downlink CCs anduplink CCs, and it is referred to as asymmetric aggregation when thenumber of downlink CCs and the number of uplink CCs are not equal.

The size of component carriers (in other words, bandwidth) may bedifferent. For example, if five component carriers are used to form 70MHz band, 5 MHz component carrier (carrier #0)+20 MHz component carrier(carrier #1)+20 MHz component carrier (carrier #2)+20 MHz componentcarrier (carrier #3)+5 MHz component carrier (carrier #4) may beaggregated together.

Hereinafter, a multiple carrier system includes the system that supportscarrier aggregation. Contiguous CA and/or non-contiguous CA may be usedin the multiple carrier system; in addition, both symmetric aggregationand asymmetric aggregation may be used in the multiple carrier system aswell. A serving cell may be defined as a component frequency band basedon multiple CC system which may be aggregated by CA. A serving cell mayinclude a primary serving cell (PCell) and a secondary serving cell(SCell). A PCell means a serving cell which provides security input andNon-Access Stratum (NAS) mobility information on Radio Resource Control(RRC) establishment or re-establishment state. Depends on the capabilityof a user equipment, at least one cell may be used together with a PCellto form an aggregation of serving cells, the cell used with a PCell isreferred to as an SCell. An aggregation of serving cells whichconfigured for a user equipment may include one PCell, or one PCelltogether with at least one SCell.

Downlink component carrier corresponding to a PCell refers to Downlink(DL) Primary Component Carrier (PCC), and uplink component carriercorresponding to a PCell refers to Uplink (UL) PCC. In addition,downlink component carrier corresponding to an SCell refers to a DLSecondary Componenent Carrier (SCC), and an uplink component carriercorresponding to an SCell refers to a UL SCC. Only DL CC or UL CC maycorrespond to a serving cell, or a DL CC and an UL CC together maycorrespond to a serving cell.

FIG. 2 is a diagram illustrating an example of a protocol structure forsupporting a multi-carrier system according to an exemplary embodimentof the present invention.

Referring to FIG. 2, common Medium Access Control (MAC) entity 210manages physical layer 220 which uses a plurality of carriers. The MACmanagement message, transmitting through a certain carrier, may beapplied to other carriers. That is, the MAC management message is amessage which controls other carriers including the certain carriermentioned above. A physical layer 220 may be operated by the TimeDivision Duplex (TDD) and/or the Frequency Division Duplex (FDD).

There are some physical control channels used in physical layer 220. Asa DL physical channel, a Physical Downlink Control Channel (PDCCH)informs to a UE with regard to resource allocation of a Paging Channel(PCH) and a Downlink Shared Channel (DL-SCH), and a Hybrid AutomaticRepeat Request (HARM) information related to a DL-SCH. The PDCCH maycarry uplink grant which informs a resource allocation of uplinktransmission to a UE. The DL-SCHO is mapping to a Physical DownlinkShared Channel (PDSCH). A Physical Control Format Indicator Channel(PCFICH), which transmits every sub-frame, informs the number of OFDMsymbols used on the PDCCHs to a user equipment. A Physical Hybrid ARQIndicator Channel (PHICH), as a DL channel, carries the HARQ ACK/NACKsignals as a response to uplink transmission. As a UL physical channel,Physical Uplink Control Channel (PUCCH) may carry UL controllinginformation such as ACK (Acknowledgement)/NACK (Non-acknowledgement) orChannel Status Information (CSI) which includes Channel QualityIndicator (CQI), Precoding Matrix Index (PMI), Precoding Type Indicator(PTI) or Rank Indication (RI). The Physical Uplink Shared Channel(PUSCH) carries the Uplink Shared Channel (UL-SCH). The Physical RandomAccess Channel (PRACH) carries random access preamble.

A plurality of the PDCCH may be transmitted in the controlled region,and a user equipment can monitor a plurality of the PDCCH. The PDCCH istransmitted on either one Control Channel Element (CCE) or anaggregation of several consecutive CCEs. The CCE is a logical allocationunit used to provide PDCCH with a code rate based on the state of radiochannel. The CCE corresponds to a plurality of Resource Element Groups.The format of the PDCCH and the number of available bits for the PDCCHare determined according to the relationship between the number of CCEsand a code rate provided by the CCEs.

Control information carried on the PDCCH is referred to as DownlinkControl Information (DCI). The following table 1 shows DCI pursuant toseveral formats.

TABLE 1 DCI Format Description 0 Used for PUSCH scheduling in uplinkcell 1 Used for one PDSCH codeword scheduling in one cell 1A Used forbrief scheduling of one PDSCH codeword in one cell or random accessprocess initialized by the PDCCH command 1B Used for a brief schedulingof one PDSCH codeword with precoding information in one cell 1C Used forone PDSCH codeword brief scheduling in one cell or the notification ofMCCH change 1D Used for a brief scheduling of one PDSCH codeword in onecell including precoding or power offset information 2 Used for thePDSCH scheduling of the user equipment configured of spartialmultiplexing mode. 2A Used for the PDSCH scheduling of the userequipment configured of large delay CDD mode 2B Used in the transmissionmode 8 (a double layer transmission, etc) 2C Used in the transmissionmode 9 (a multi layer transmission) 2D Used in the transmission mode 10(CoMP) 3 Used for the tramission of TPC commands for PUCCH and PUSCHincluding 2-bit power adjustment 3A Used for the tramission of TPCcommands for PUCCH and PUSCH including single-bit power adjustment 4Used for the PUSCH scheduling in the uplink multi-antenna porttransmission cell

Referring to Table 1, There are DCI formats such as format 0 used forthe PUSCH scheduling in uplink cell, format 1 used for one PDSCHcodeword scheduling in one cell, format 1A used for compact schedulingof one PDSCH codeword, format 2 used for the PDSCH scheduling inclosed-loop spatial multiplexing mode, format 2B used for the PDSCHscheduling in open-loop spatial multiplexing mode, format 2B used in thetransmission mode 8, format 2C used in the transmission mode 9, format2D used in the transmission mode 10, format 3 and 3A used for the uplinktransmission of TPC commands for the PUCCH and the PUSCH, and format 4used for the the PUSCH scheduling in the uplink multi-antenna porttransmission cell.

Each field of DCI is sequentially mapped to n number of information bitsa₀ or a_(n-1). For example, the DCI is mapped to a total length of 44bits of information bits, each field of DCI is mapped sequentially to a₀or a₄₃. DCI formats 0, 1A, 3, 3A may have the same payload size. DCIformat 0, 4 may be referred to as the Uplink grant (UL grant).

Cross-carrier scheduling is a scheduling method capable of performingresource allocation of a PDSCH transmitted by using a different carrierthrough a PDCCH transmitted through a specific CC and/or resourceallocation of a PUSCH transmitted by using another CC other than a CCbasically linked to the specific CC. That is, the PDCCH and the PDSCHmay be transmitted through different DL CCs, and the PUSCH may betransmitted through a UL CC other than a UL CC linked to a DL CC onwhich a PDCCH including a UL grant is transmitted.

During cross-carrier scheduling, a user equipment only receivesscheduling information (such as UL grant) through a serving cell (orCC). Hereinafter, a serving cell (or CC) performing cross-carrierscheduling may refer to scheduling cell (or CC), and a serving cellbeing scheduled by scheduling cell may refer to scheduled cell (or CC).Scheduling cell may refer to ordering cell, and scheduled cell may referto following serving cell. For example, a scheduled cell may bescheduled by a scheduling cell. Scheduling information for the scheduledcell may be received through the scheduling cell.

As such, in a system supporting the cross-carrier scheduling, a carrierindicator is necessary to report which DL CC/UL CC was used to transmitthe PDCCH/EPDCCH which indicates the PDSCH/PUSCH transmission. A fieldincluding the carrier indicator is hereinafter called a carrierindication field (CIF). Hereinafter, configuration of CIF may mean thatconfiguration of cross-carrier scheduling.

The aforementioned cross-carrier scheduling may be classified into theDL cross-carrier scheduling and UL cross-carrier scheduling. The DLcross-carrier scheduling implies a case where the CC for transmittingthe PDCCH/EPDCCH including resource allocation information for the PDSCHtransmission and other information is different from a CC fortransmitting the PDSCH. The UL cross-carrier scheduling implies a casewhere a CC for transmitting the PDCCH/EPDCCH including a UL grant forthe PUSCH transmission is different from the DL CC linked to the UL CCfor transmitting the PUSCH.

FIG. 3 is a diagram illustrating an example of a radio frame structureaccording to an exemplary embodiment of the present invention. Thediagram illustrates a FDD radio frame structure and a TDD radio framestructure.

Referring to FIG. 3, one radio frame includes 10 subframes, and onesubframe includes 2 consecutive slots.

In the FDD, both carrier used for UL transmission and carrier used forDL transmission exist, and UL transmission and DL transmission may beperformed simultaneously in one cell.

In the TDD, on one cell basis, UL transmission and DL transmission canalways distinguished in time. Because a same carrier is used for both ULtransmission and DL transmission, a base station and user equipmentrepeatedly switches between the transmission mode and the receptionmode. In the TDD, special subframe may be placed to provide a guard timewhich is for switching mode between the transmission and the reception.Special subframe, as shown, includes a downlink pilot time slot (DwPTS),a guard period (GP), and an uplink pilot time slot (UpPTS). The DwPTS isused in the UE for initial cell search, synchronization, or channelestimation. The UpPTS is used in the BS for channel estimation anduplink transmission synchronization of the UE. The GP is needed to avoidinterference between an uplink and a downlink, and during the GP, no ULtransmission and DL transmission occurs.

Table 2 shows an example of UL-DL configuration of radio frame. UL-DLconfiguration defines reserved subframe for UL transmission or reservedsubframe for DL transmission. That is, UL-DL configuration informs therules how the uplink and the downlink are allocated (or reserved) inevery subframe of one radio frame.

TABLE 2 Uplink- downlink configu- Switch-point Subframe number rationperiodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U U D S U U U 1 5 ms D S UU D D S U U D 2 5 ms D S U D D D S U D D 3 10 ms D S U U U D D D D D 410 ms D S U U D D D D D D 5 10 ms D S U D D D D D D D 6 5 ms D S U U U DS U U D

In Table 2, ‘D’ denotes a DL subframe, ‘U’ denotes a UL subframe, and‘S’ denotes a special subframe. As shown to Table 2, subframe 0 and 5are always allocated to DL transmission, and subframe 2 is alwaysallocated to UL-transmission. As shown to Table 2, each UL-DLconfiguration has a different number and position of DL subframe and ULsubframe in one radio frame. Through diverse UL-DL configuration, theamount of resource allocated to UL-DL transmission may be givenasymmetrically. To avoid severe interference between UL and DL amongcells, neighboring cells generally have same UL-DL configuration.

The point changing from DL to UL or the point changing from UL to DL isreferred to as the switching point. The switch-point periodicity, whichis either 5 ms or 10 ms, means a repeating period of the same changingaspect between the UL subframe and DL subframe. For example, referringto the UL-DL configuration 0, subframe from 0 to 4 changes D→S→U→U→U,subframe from 5 to 9 changes, as same as before, D→S→U→U→U. Since onesubframe is 1 ms, the switch-point periodicity is 5 ms. That is, theswitch-point periodicity is shorter than the length of one radio frame(10 ms), the changing aspect in the radio frame is repeated for onetime.

The UL-DL configuration in above Table 2 may be transmitted from a basestation to a user equipment through system information. The base stationmay inform a UL-DL allocation status change in a radio frame to a UE bytransmitting the index of the UL-DL configuration whenever the UL-DLconfiguration changes. Or the UL-DL configuration may be controlinformation which is transmitted to every UE in the cell throughbroadcast channel.

Automatic Repeat Request (ARQ) is one of the technologies for increasingreliability of radio communication. According to ARQ, a transmitterretransmits a data signal when a receiver fails to receive the datasignal. Also, Hybrid Automatic Repeat Request (HARQ) is a combination ofForward Error Correction (FEC) and ARQ. A receiver that uses an HARQprocess basically attempts error correction with respect to a receiveddata signal, and determines whether to execute retransmission using anerror detection code. The error detection code may use Cyclic RedundancyCheck (CRC). When an error of a data signal is not detected through theCRC detection process, the receiver determines that decoding of the datasignal is successful. If the decoding of the data signal is determinedto be successful, the receiver transmits an Acknowledgement (ACK) signalto the transmitter. When an error of a data signal is detected throughthe CRC detection process, the receiver determines that decoding of thedata signal is unsuccessful. If the decoding of the data signal isdetermined to be unsuccessful, the receiver stores a part or theentirety of the received data signal in a soft buffer, and transmits aNon-Acknowledgement (NACK) signal to the transmitter. The transmittermay retransmit a data signal when the NACK signal is received. Thereceiver receives a packet retransmitted from the transmitter, andcombines the stored data signal and the retransmitted data signal, so asto increase a probability of success of decoding. Soft channel bits areallocated to each HARQ process, and the soft channel bits may be storedin the soft buffer. Therefore, a space of the soft buffer needs to bereserved for the HARQ process related to a data signal that a UE failsto correctly decode. When the soft buffer is fully utilized, or thespace is insufficient, the UE may not execute the HARQ process,smoothly. Hereinafter, storing a HARQ process in the soft bufferincludes storing soft channel bits allocated to the HARQ process in thesoft buffer.

First, downlink (DL) HARQ will be described. When a base stationtransmits, to a UE, a DL grant which is PDSCH scheduling informationthrough a PDCCH/EPDCCH and transmits the PDSCH, the UE transmits,through a PUCCH at a predetermined timing, a HARQAcknowledgement/Non-acknowledgement (ACK/NACK) with respect to a DL-SCHTransport Block (TB) that is received through the PDSCH. The DL HARQindicates a process of repeating the above described process in apredetermined period of time, until the base station receives an ACKsignal from the UE. According to the current standard, when PUSCHtransmission is indicated in an Uplink (UL) subframe that is configuredfor transmission of other Uplink Control Information (UCI) including anHARQ ACK/NACK signal, the UCI including the HARQ ACK/NACK signal may betransmitted together on the indicated PUSCH under a predetermined rule.Hereinafter, it is described that the HARQ ACK/NACK signal of the DLHARQ is transmitted on a PUCCH, and exemplary embodiments of the presentinvention include that the HARQ ACK/NACK signal is transmitted on aPUSCH based on whether a PUSCH is transmitted on a corresponding ULsubframe.

For the FDD, when the UE detects a PDSCH transmission for thecorresponding UE in a subframe n−4, the UE transmits a HARQ responseresponsive to the PDSCH transmission in a subframe n.

For TDD, when PDSCH transmission indicated by detection of acorresponding PDCCH/EPDCCH exists in a subframe n−k, or when aPDCCH/EPDCCH indicating Semi-Persistent Scheduling (SPS) release existsin the subframe n−k, the UE transmits a HARQ response responsive to thePDCCH/EPDCCH in a subframe n. For TDD or some TDD configured carrieraggregations (CAs), DL HARQ ACK/NACK transmission timings may be listedas shown in Table 3.

TABLE 3 Downlink association set index K: {k₀, k₁, . . . , K_(M−1)} forTDD and some TDD configured CAs UL-DL Configu- Subframe n rations 0 1 23 4 5 6 7 8 9 0 — — 6 — 4 — — 6 — 4 1 — — 7, 6 4 — — — 7, 6 4 — 2 — — 8,7, — — — — 8, 7, — — 4, 6 4, 6 3 — — 7, 6, 11 6, 5 5, 4 — — — — — 4 — —12, 8, 6, 5, — — — — — — 7, 11 4, 7 5 — — 13, 12, 9, — — — — — — — 8, 7,5, 4, 11, 6 6 — — 7 7 5 — — 7 7 —

In Table 3, n denotes a subframe number having an index n, and a “DLsubframe set” associated with a subframe of the corresponding number isdetermined by K={k₀, k₁, . . . , K_(M-1)}. n−k denotes an index of asubframe that is located k subframes before the subframe n, andindicates a DL subframe associated with the subframe n. The associatedDL subframe indicates a subframe that delivers a PDSCH which is thebasis of the determination on an ACK/NACK signal. M denotes the numberof elements of a set K defined in table 3, and indicates the number ofDL subframes associated with the subframe n.

For example, when UL-DL configuration corresponding to a single servingcell is 1, M of a DL subframe set K associated with a subframe 2 is 2(M=2), k₀=7, and k₁=6. Therefore, DL subframes associated with thesubframe 2 of the corresponding serving cell are a subframe 5 (2-k₀) anda subframe 6 (2-k₁) of the previous radio frame because each radio framehas ten subframes from subframe 0 to subframe 9.

FIG. 4 illustrates a case of an inter-band CA of serving cells havingdifferent TDD UL-DL configurations.

Referring to FIG. 4, component carriers that configure a CA with a UEare CC1 and CC2, the CC1 may be configured as UL-DL configuration #0 andCC2 may be configured as UL-DL configuration #5, for the purpose oftraffic adaption (semi-static) and avoidance of interference betweenheterogeneous networks. For example, to avoid an interference issue withother TDD systems (for example, TDS-CDMA, WiMAX, and the like) thatco-exist in an identical band, different UL-DL configurations may berequired in an inter-band CA. In addition, when a UL-DL configurationincluding a large number of UL subframes is applied to a high frequencyband, and a UL-DL configuration including a large number of DL subframesis applied to a low frequency band, it may be helpful for the coverageenhancement.

For the TDD, when a UE is configured with one or more serving cells, atleast two serving cells have different UL-DL configurations, and acorresponding serving cell is a Primary Service Cell (PCell), a UL-DLconfiguration of the corresponding PCell is a DL reference UL-DLconfiguration for the PCell. Here, the DL reference UL-DL configurationindicates a UL-DL configuration used as a reference for a DL HARQ timingof a corresponding serving cell.

Meanwhile, for the TDD, when a UE is configured with two or more servingcells, at least two serving cells have different UL-DL configurations,and a corresponding serving cell is a Secondary Serving Cell (SCell), aDL reference UL-DL configuration for the corresponding SCell is as shownin the following Table 4.

TABLE 4 Set (Primary cell UL-DL configuration, DL-reference UL- #Secondary cell UL-DL configuration) DL configuration Set 1 (0, 0) 0 (1,0), (1, 1), (1, 6) 1 (2, 0), (2, 2), (2, 1), (2, 6) 2 (3, 0), (3, 3),(3, 6) 3 (4, 0), (4, 1), (4, 3), (4, 4), (4, 6) 4 (5, 0), (5, 1), (5,2), (5, 3), 5 (5, 4), (5, 5), (5, 6) (6, 0), (6, 6) 6 Set 2 (0, 1),(6, 1) 1 (0, 2), (1, 2), (6, 2) 2 (0, 3), (6, 3) 3 (0, 4), (1, 4), (3,4), (6, 4) 4 (0, 5), (1, 5), (2, 5), (3, 5), 5 (4, 5), (6, 5) (0, 6) 6Set 3 (3, 1), (1, 3) 4 (3, 2), (4, 2), (2, 3), (2, 4) 5 Set 4 (0, 1),(0, 2), (0, 3), (0, 4), 0 (0, 5), (0, 6) (1, 2), (1, 4), (1, 5) 1 (2, 5)2 (3, 4), (3, 5) 3 (4, 5) 4 (6, 1), (6, 2), (6, 3), (6, 4), (6, 5) 6 Set5 (1, 3) 1 (2, 3), (2, 4) 2 (3, 1), (3, 2) 3 (4, 2) 4

In Table 4, based on a pair of a PCell UL-DL configuration and an SCellUL-DL configuration, the DL reference UL-DL configuration for the SCellmay be indicated.

For example, when the pair of the PCell UL-DL configuration and theSCell UL-DL configuration of Table 4 belongs to Set 1, the DL referenceUL-DL configuration for the SCell applies a DL HARQ timing based on theDL reference UL-DL configuration for Set 1. In this instance, it isirrespective of a scheduling method.

In a case in which self-scheduling is set for a UE, when the pair of thePCell UL-DL configuration and the SCell UL-DL configuration belongs toSet 2 or Set 3, a DL reference UL-DL configuration of Set 2 or Set 3 maybe used. Here, when self-scheduling is set for the UE, it indicates thatthe UE is not set to monitor a PDCCH/EPDCCH of another serving cell forscheduling of a corresponding serving cell.

In a case in which cross-carrier scheduling is set for a UE, when thepair of the PCell UL-DL configuration and the SCell UL-DL configurationbelongs to Set 4 or Set 5, a DL reference UL-DL configuration of Set 4or Set 5 may be used. Here, when cross-carrier scheduling is set for theUE, it indicates that the UE is set to monitor a PDCCH/EPDCCH of anotherserving cell for scheduling of a corresponding serving cell.

More specifically, the DL reference UL-DL configuration of Set 1 may beapplied when a corresponding pair is satisfied, irrespective of whethera Carrier Indicator Field (CIF) indicating a carrier associated withscheduling is configured. Further, Set 2/Set 3 may be applied to onlyUEs for which the CIF is not configured, and Set 4/Set 5 may be appliedto only UEs for which the CIF is configured.

An ACK/NACK signal with respect to a PDCCH/EPDCCH that indicates a PDSCHor SPS release corresponding to each of a plurality of serving cells ofa CA may be transmitted at the above described HARQ timing.

For implementing a system conforming to the 3^(rd) GenerationPartnership Project (3GPP) Long Term Evolution (LTE) Release (Rel)-11, amethod of storing soft channel bits may be used as follows.

When a UE is configured with more than one serving cell, with respect toeach serving cell, upon failure of decoding of a code block of aTransport Block (TB), the UE stores received soft channel bits for atleast K_(MIMO). min(M_(DL) _(_) _(HARQ), M_(limit)) TBs. The UE maystore received soft channel bits corresponding to at least a range ofW_(k)W_(k)=1, . . . , W_(mod(k+n) _(SB) _(−1,N) _(cb) ₎. Here, w_(k)denotes a soft bit index. To determine k, the UE may need to assign apriority for storing soft channel bits corresponding to a lower k value.

In addition, n_(SB) denotes the number of received soft channel bitsthat the UE stores per code block unit, and may be calculated based onthe following Equation 1.

$\begin{matrix}{n_{SB} = {\min( {N_{cb},\lfloor \frac{N_{soft}^{\prime}}{C \cdot N_{cells}^{DL} \cdot K_{MIMO} \cdot {\min( {M_{DL\_ HARQ},M_{limit}} )}} \rfloor} )}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$

Here, C denotes the number of code blocks forming a single TB, N_(cb)denotes a soft buffer size of a code block, and K_(MIMO) is 2 when a UEcorresponds to a MIMO Transport Mode (TM), and is 1 for the rest cases.M_(limit) is integer 8, and N^(DL) _(cells) denotes the number ofserving cells configured for the UE.

M_(DL) _(_) _(HARQ) denotes the maximum number of DL HARQ processes, andmay be determined based on the following criteria.

For the FDD, a maximum of eight DL HARQ processes exists for eachserving cell.

For TDD, when a UE is configured with a single serving cell, or when theUE is configured with one or more serving cells and TDD UL-DLconfigurations of all of the serving cells are identical, the maximumnumber of DL HARQ processes for each serving cell may be determinedbased on the TDD UL-DL configuration, which is as shown in Table 5.

TABLE 5 TDD UL/DL Maximum number of configuration HARQ processes 0 4 1 72 10 3 9 4 12 5 15 6 6

For TDD, when a UE is configured with more than one serving cells andTDD UL-DL configurations of at least two serving cells are different,the maximum number of DL HARQ processes is determined based on the abovedescribed Table 5. In this instance, “TDD UL-DL configuration” may referto a “DL reference UL-DL configuration” determined based on the abovedescribed Table 4. For example, when a TDD UL-DL configuration for aserving cell is 6, and a DL reference UL-DL configuration of the servingis 1, the number of DL HARQ processes is determined to be 7 based on theconfiguration index 1, which is the DL reference UL-DL configuration forthe corresponding serving cell.

A dedicated broadcast HARQ process is not counted when the maximumnumber of HARQ processes is calculated, for both the FDD and the TDD.

N′_(soft) in Equation 1 denotes a total number of soft channel bits of aUE. The total number of soft channel bits of the UE is determined basedon a UE category. The category of the UE may be indicated by an RRCmessage. For example, ue-Category field (or ue-Category-v1020 field) ofthe RRC message may indicate the category of the UE. The ue-Categoryfield (or ue-Category-v1020 field) may be included in aUE-EUTRA-Capability information element of an RRC message.

In particular, ue-Category field includes parameters that define acombined UL and DL capability. The parameters may be determined based ona category of a UE. For example, ue-Category field may include parametervalues associated with a DL physical layer, and the parameter values maybe listed as shown in Table 6.

TABLE 6 Maximum number of Maximum number of bits Maximum number ofDL-SCH transport of a DL-SCH trans- Total number supported layers forblock bits received port block received of soft spatial multiplexing UECategory within a TTI (Note) within a TTI channel bits in DL Category 110296 10296 250368 1 Category 2 51024 51024 1237248 2 Category 3 10204875376 1237248 2 Category 4 150752 75378 1827072 2 Category 5 299552149776 3667200 4 Category 6 301504 149776 (4 layers) 3654144 2 or 475376 (2 layers) Category 7 301504 149776 (4 layers) 3854144 2 or 475376 (2 layers) Category 8 2998560 299856 35982720 8

Here, “Total number of soft channel bits” denotes a total number of softchannel bits for storage according to a category of a UE. This maycorrespond to the above described N′_(soft).

Hereinafter, exemplary embodiments for a TDD-FDD CA (or a TDD-FDD jointoperation) that supports a CA of an FDD carrier and a TDD carrier willbe described.

FIG. 5 illustrates an example of an FDD-TDD CA according to an exemplaryembodiment of the present invention.

Referring to FIG. 5, a legacy TDD terminal 520 receives a wirelesscommunication service only through a TDD band, and a legacy FDD terminal540 receives a wireless communication service only through an FDD band.Further, an FDD-TDD CA capable UE 500 receives a wireless communicationservice through an FDD band and a TDD band, and may also receive aCA-based wireless communication service through a TDD band carrier andan FDD band carrier.

For the above described TDD-FDD CA, for example, the followingdeployment scenarios may be considered.

For example, an FDD base station and a TDD base station are co-locatedin an identical place (for example, CA scenarios 1 through 3), or theFDD base station and the FDD base station are not co-located butconnected through an ideal backhaul (for example, CA scenario 4).

As another example, the FDD base station and the TDD base station arenot co-located and connected through a non-ideal backhaul (for example,small cell scenarios 2a and 2b and a macro-macro scenario).

However, for the TDD-FDD CA, it may be desirable that the TDD basestation and the FDD base station are connected through an idealbackhaul, and the TDD cell and the FDD cell are synchronized foroperation.

In addition, for the TDD-FDD CA, the following prerequisites may beconsidered.

First, UEs that support the FDD-TDD CA may access a legacy FDD singlemode carrier and a legacy TDD single mode carrier.

Second, legacy FDD terminals and UEs that support the TDD-FDD CA maycamp on and be connected to an FDD carrier which is a part of an FDD/TDDnetwork that executes a joint operation.

Third, legacy TDD terminals and UEs that support the TDD-FDD CA may campon and be connected to a TDD carrier which is a part of the FDD/TDDnetwork that executes a joint operation.

Fourth, network architecture enhancement for facilitating the FDD-TDDCA, for example, with respect to the non-ideal backhaul, may beconsidered. However, keeping the minimal network architecture changesshould be considered since it is essential from the perspective of anoperator.

In addition, the following capabilities of a UE may be considered whenthe UE supports the TDD-FDD CA.

FIG. 6 illustrates examples of capabilities of a UE for a TDD-FDD CAaccording to an exemplary embodiment of the present invention.

In FIG. 6, an example (a) illustrates that a UE supports a CA of a TDDcarrier and an FDD carrier, an example (b) illustrates that a UEsupports a CA of a TDD carrier and an FDD DL carrier, and an example (c)illustrates that a UE supports a CA of a DL subframe of a TDD carrierand an FDD carrier.

As mentioned above, a UE may support different types of TDD-FDD jointCAs, and also, may perform simultaneous reception (that is, DLaggregation) in FDD and TDD carriers. Second, a UE may performsimultaneous transmission (that is, UL aggregation) in FDD and TDDcarriers. Third, a UE may perform simultaneous transmission andreception (that is, full duplex) in FDD and TDD carriers.

In the above described TDD-FDD CA, the maximum number of aggregatedComponent Carriers (CCs) may be 5, for example. In addition, anaggregation of different UL-DL configurations for TDD carriers ofdifferent bands may be supported.

The FDD-TDD CA capable UE may support a TDD-FDD DL CA and may notsupport a TDD-FDD UL CA. The FDD-TDD CA capable UE may support at leasta TDD-FDD DL CA, but may or may not support a TDD-FDD UL CA.

Further, regardless of whether or not a TDD-FDD UL CA is configured fora UE, transmission of at least one PUCCH (and PUSCH) may be supported ononly a PCell (on PCell-only). Further, a transmission of a PUCCH (andPUSCH) may be supported on an SCell, in addition to the PCell.

When a PUCCH is transmitted on only a PCell, the following operationsmay be executed. First, for a PDSCH/PUSCH transmitted on a PCell, ascheduling/HARQ timing is based on a timing of the PCell, irrespectiveof whether the PCell is a TDD carrier or an FDD carrier. Second, for aPUSCH transmitted on an SCell based on self-scheduling, ascheduling/HARQ timing is based on a timing of the SCell, irrespectiveof whether the SCell is a TDD carrier or an FDD carrier. When the PCellis an FDD carrier, and the SCell is a TDD carrier, for a PDSCHtransmitted on the SCell based on self-scheduling, a HARQ timing isbased on a timing of the PCell.

A UE may configure dual connectivity through two or more base stationsamong base stations that configure at least one serving cell. The dualconnectivity is an operation in which a corresponding UE utilizes radioresources provided by at least two different network points (forexample, a macro base station and a small base station) in a radioresource control connection (RRC_CONNECTED) mode. The at least twodifferent network points may be connected through a non-ideal backhaul.Further, one of the at least two different network points may bereferred to as a macro base station (or a master base station or ananchor base station), and the remaining base stations may be referred toas small base stations (or secondary base stations, assisting basestations, or slave base stations).

A UE may support a TDD-FDD joint operation when a CA and/or dualconnectivity is configured for the UE. Hereinafter, although a case inwhich a CA is configured for a UE will be described, aspects of thepresent invention may be applied for a case in which the dualconnectivity is configured for the UE.

The UEs conforming to LTE release 11 or older releases detect themaximum number of DL HARQ processes, based on a DL HARQ timingassociated with a TDD UL-DL configuration or a DL HARQ timing associatedwith an FDD, and may store, in a soft buffer, soft channel bits acquiredusing the maximum number of DL HARQ processes as an input value. Morespecifically, an existing DL HARQ method only takes into consideration aCA of carriers having an identical TDD UL-DL configuration or differentTDD UL-DL configurations, and a CA of FDD carriers, and stores softchannel bits of a UE based on the maximum number of DL HARQ processes.However, when a TDD-FDD CA is configured for a UE, different DL HARQtimings may be applied to different serving cells (a PCell and anSCell), and in this case, the maximum number of DL HARQ processes may bedetected for each serving cell. More specifically, the maximum number ofDL HARQ processes, which is a main factor that a UE needs to take intoconsideration when storing soft channel bits in the TDD-FDD CA, may bedifferent for each configured serving cell. Thus, there is desire for asoft channel bits storing method that may be effectively applied to alimited soft buffer of the UE.

The TDD-FDD CA may be classified into four cases, based on a TDD-FDDtype and a scheduling mode of a PCell and an SCell, as shown in thefollowing Table 7.

TABLE 7 Type Scheduling mode Case 1 PCell(TDD)-SCell(FDD) selfscheduling Case 2 PCell(FDD)-SCell(TDD) self scheduling Case 3PCell(TDD)-SCell(FDD) Cross-carrier scheduling Case 4PCell(FDD)-SCell(TDD) Cross-carrier scheduling

The soft channel bits storing methods may be applied to the four cases.For example, one of the channel bits storing methods may be applied toall of the four cases. As another example, one of the methods may beapplied to some cases, and another method may be applied to theremaining cases. For example, one of the methods may be applied to Case1, and another method may be applied to the remaining cases.

The methods provided herein may improve the overall performance for a DLHARQ process of a UE, by applying the methods to 256 QAM, instead ofapplying to a Modulation Coding Scheme (MCS). Therefore, the softchannel bits storing methods may be applied to a new UE category or UEsthat support 256 QAM, in addition to the TDD-FDD CA.

Hereinafter, the soft channel bits storing methods according toexemplary embodiments of the present invention will be described indetail.

Method 1: A Soft Channel Bits Storing Method Based on the Maximum Numberof DL HARQ Processes by Taking into Account a DL Subframe that isAvailable for DL PDSCH Scheduling

The maximum number of DL HARQ processes may be calculated based on a TDDUL-DL configuration (or a DL reference UL-DL configuration), asdescribed with reference to Table 5, and soft channel bits may be storedusing the above mentioned Equation 1. When such method is applied to aUE for which a TDD-FDD CA is configured, the performance of the UE maybe deteriorated. In order to address such problem, the present methodprovides a soft channel bits storing method that may be applicable to aTDD-FDD CA, by changing the definition of M_(DL) _(_) _(HARQ) applied toEquation 1.

FIG. 7 illustrates examples of the maximum number of DL HARQ processeswhen a TDD-FDD CA is configured for a UE according to an exemplaryembodiment of the present invention. In FIG. 7, it is assumed that aPCell is an FDD carrier, an SCell is a TDD carrier of TDD UL-DLconfiguration 0, and a scheduling mode is a self scheduling mode (thatis, Case 1 of Table 7).

Referring to FIG. 7, for an SCell, an FDD DL HARQ timing may be applied.Here, alt 1 corresponds to a case that uses an existing method to countthe number of DL HARQ processes corresponding to an FDD DL HARQ timingapplied to an SCell, and alt 2 corresponds to a case that counts thenumber of DL HARQ processes by taking into consideration a DL subframethat is available for DL PDSCH scheduling. For alt1 and alt2, M_(DL)_(_) _(HARQ) may be determined as shown below.

TABLE 8 M_(DL) _(—) _(HARQ) alt 1 8(based on FDD HARQ timing) alt 24(actual DL sub frame for PDSCH transmission)

A performance gain that a UE may obtain may be compared between caseswhen M_(DL) _(_) _(HARQ) of alt 1 is 8 and when M_(DL) _(_) _(HARQ) ofalt 2 is 4, as shown below.

FIG. 8 illustrates a comparison of a performance of a UE with respect toa single code block, based on an M_(DL) _(_) _(HARQ) value according toan exemplary embodiment of the present invention. FIG. 8 illustrates adifference in performance between alt 1 (M_(DL) _(_) _(HARQ)=8) and alt2(M_(DL) _(_) _(HARQ)=4), based on different Transport Block Sizes(TBSs) (that is, different MCS levels). In FIG. 8, it is assumed that anFDD(PCell)-TDD(SCell) CA and self-scheduling is configured for a UE,K_(MIMO)=2, two Cell-specific Reference Signal (CRS) APs are configured,a system bandwidth is 50 PRB (physical resource block)s, and a UEcategory is 3. An effect on performance of a UE may be analyzed bycomparing, between alt 1 and alt 2, an amount of soft channel bits thata UE may store based on each MCS level. n_(sb) denotes the number ofsoft channel bits that a UE may store for each code block unit, asdescribed above. That is, n_(sb) is a size of soft channel bits that aUE may be allocate (or store) for a single code block.

Referring to FIG. 8, n_(sb) is 17376 for alt 1 and alt 2 when an MCSlevel is 3. n_(sb) is 18396 for alt 1 and alt 2 when the MCS level is 7.n_(sb) is 12888 for alt 1 and n_(sb) is 17760 for alt 2, when the MCSlevel is 11. n_(sb) is 7732 for alt 1 and n_(sb) is 15465 for alt 2,when the MCS level is 15. n_(sb) is 7732 for alt 1 and n_(sb) is 15465for alt 2, when the MCS level is 17. n_(sb) is 5523 for alt 1 and n_(sb)is 11046 for alt 2, when the MCS level is 20. n_(sb) is 3514 for alt 1and n_(sb) is 7029 for alt 2, when the MCS level is 26. Therefore,generally, alt 2 that has M_(DL) _(_) _(HARQ) of 4 has a relativelylarger amount of soft channel bits than alt 1 that has M_(DL) _(_)_(HARQ) of 8. A UE may improve error correction capability for a TBwhere a corresponding code block belongs, through the larger amount ofsoft channel bits.

In particular, in a case of a high MCS level (for example, 20 or 26),for alt 1, there is a case that n_(sb) is even smaller than a size ofsystematic bits corresponding to original information. This may have anadverse effect on the error correction capability of the UE. Inaddition, a smaller information size is stored in a UE and thus, atransmission rate may be deteriorated even in a good channel. Inaddition, there is a drawback in that an information size that a basestation transmits through rate matching for a single code block is notsufficiently stored and thus, the base station may occupy unnecessaryresources.

Therefore, when the maximum number of DL HARQ processes is calculatedbased on a DL subframe that is actually available for DL PDSCHscheduling on an SCell, as illustrated in Method 1 according to anexemplary embodiment of the present invention, the UE may allocate arelatively larger number of soft channel bits per code block and mayimprove error correction capability.

The maximum number of DL HARQ processes for each TDD UL-DL configurationof the SCell may be listed as below.

TABLE 9 Case 1: FDD(PCell)-TDD(SCell) CA with self-scheduling M_(DL)_(—) _(HARQ) SCell PCell SCell PCell alt 1 alt 2 FDD TDD UL-DLconfiguration 0 8 8 4 FDD TDD UL-DL configuration 1 8 8 5 FDD TDD UL-DLconfiguration 2 8 8 6 FDD TDD UL-DL configuration 3 8 8 5 FDD TDD UL-DLconfiguration 4 8 8 6 FDD TDD UL-DL configuration 5 8 8 7 FDD TDD UL-DLconfiguration 6 8 8 4

In Table 9, alt 1 corresponds to a case that uses an existing method tocount the number of DL HARQ processes corresponding to an FDD DL HARQtiming applied to an SCell when the FDD DL HARQ timing is applied to theSCell, and alt 2 corresponds to a case that counts the number of DL HARQprocesses by taking into consideration a DL subframe that is availablefor DL PDSCH scheduling. In this instance, both the PCell and the SCellto which alt 1 is applied have M_(DL) _(_) _(HARQ) of 8. However, for acase of the SCell to which alt 2 is applied, M_(DL) _(_) _(HARQ) is 4when the TDD UL-DL configuration of the SCell is 0, M_(DL) _(_) _(HARQ)is 5 when the TDD UL-DL configuration of the SCell is 1, M_(DL) _(_)_(HARQ) is 6 when the TDD UL-DL configuration of the SCell is 2, M_(DL)_(_) _(HARQ) is 5 when the TDD UL-DL configuration of the SCell is 3,M_(DL) _(_) _(HARQ) is 6 when the TDD UL-DL configuration of the SCellis 4, M_(DL) _(_) _(HARQ) is 7 when the TDD UL-DL configuration of theSCell is 5, and M_(DL) _(_) _(HARQ) is 4 when the TDD UL-DLconfiguration of the SCell is 6.

Therefore, when an M_(DL) _(_) _(HARQ) value of Equation 1 is determinedbased alt 2 of Method 1, the soft buffer of the UE may be effectivelyutilized. That is, when the M_(DL) _(_) _(HARQ) value is applied as avalue based on alt 2 of Table 9, the UE divides a total soft channel bitsize N′_(soft) corresponding to a category of the UE for each servingcell as shown in Equation 1. A size identical for each serving cell isdivided by a MIMO transmission mode K_(MIMO), the maximum number of DLHARQ processes M_(DL) _(_) _(HARQ), and the number of code blocks C foreach TB, so as to determine a size of soft channel bits that a singlecode block may occupy.

Although Method 1 has been illustrated based on Case 1 of Table 7,Method 1 may be applied to other Cases 2, 3, and 4. However, Cases 2, 3,and 4 may provide lower efficiency than Case 1.

Method 2: A Method of Selectively Applying Method 1 and the ExistingMaximum Number of HARQ Processes

The current standard defines the maximum number of DL HARQ processesthat may be provided based on a DL HARQ timing corresponding to an FDDcarrier and the maximum number of DL HARQ processes that may be providedbased on a DL HARQ timing corresponding to seven TDD UL-DLconfigurations of a TDD carrier. This may be listed, as shown below.

TABLE 10 TDD UL/DL Maximum number of configuration DL HARQ processes 0 41 7 2 10 3 9 4 12 5 15 6 6 FDD 8

In table 10, the maximum number of DL HARQ processes may correspond toan M_(DL) _(_) _(HARQ) value and thus, by selectively using the existingmaximum number of DL HARQ processes of Table 10 and the maximum numberof DL HARQ processes determined by taking into account a DL subframethat is actually available for DL PDSCH scheduling on an SCell accordingto alt 2 of Method 1, the soft channel bits that are stored in a UE maybe controlled.

(1) Method 2-1

For the values that are different from existing values from among newM_(DL) _(_) _(HARQ) values (alt 2) generated through Method 1, thesmallest value may be selected from among existing values greater than acorresponding new M_(DL) _(_) _(HARQ). This may be listed, as shownbelow.

TABLE 11 Case 1: FDD(PCell)-TDD(SCell) CA with self-scheduling M_(DL)_(—) _(HARQ) SCell PCell SCell PCell alt 2 alt 2-1 FDD TDD UL-DLconfiguration 0 8 4 4 FDD TDD UL-DL configuration 1 8 5 6 FDD TDD UL-DLconfiguration 2 8 6 6 FDD TDD UL-DL configuration 3 8 5 6 FDD TDD UL-DLconfiguration 4 8 6 6 FDD TDD UL-DL configuration 5 8 7 7 FDD TDD UL-DLconfiguration 6 8 4 4

In Table 11, alt 2-1 denotes an M_(DL) _(_) _(HARQ) value according toMethod 2-1.

For example, when the TDD UL-DL configuration of the SCell is 0, M_(DL)_(_) _(HARQ) for the corresponding SCell is 4. When the TDD UL-DLconfiguration of the SCell is 1, M_(DL) _(_) _(HARQ) for thecorresponding SCell is 6. When the TDD UL-DL configuration of the SCellis 3, M_(DL) _(_) _(HARQ) for the corresponding SCell is 6. Method 2-1may be expressed by the following construction.

TABLE 12 if M′_(DL) _(—) _(HARQ)∈ {4,6,7,8,9,10,12,15}, M_(DL) _(—)_(HARQ)=M′_(DL) _(—) _(HARQ) else M_(DL) _(—) _(HARQ) such that M′_(DL)_(—) _(HARQ) < minimum value of {4,6,7,8,9,10,12,15}

Here, M′_(DL) _(_) _(HARQ) denotes a new M_(DL) _(_) _(HARQ) valuegenerated through Method 1, and M_(DL) _(_) _(HARQ) denotes a finalM_(DL) _(_) _(HARQ) value obtained based on Method 2-1.

The above described Method 2-1 may be advantageous in that it may reusean M_(DL) _(_) _(HARQ) value which has been defined and embodied, butmay have a non-optimal soft buffer partition. A soft buffer ispartitioned by taking into consideration a higher M_(DL) _(_) _(HARQ)value than it really is and thus, the number of soft channel bits for asingle code block may be smaller than Method 1.

(2) Method 2-2

For the values that are different from existing values from among newM_(DL) _(_) _(HARQ) values (alt 2) generated through Method 1, thelargest value may be selected from among existing values smaller than acorresponding new M_(DL) _(_) _(HARQ). This may be listed, as shownbelow.

TABLE 13 Case 1: FDD(PCell)-TDD(SCell) CA with self-scheduling M_(DL)_(—) _(HARQ) SCell PCell SCell PCell alt 2 alt 2-2 FDD TDD UL-DLconfiguration 0 8 4 4 FDD TDD UL-DL configuration 1 8 5 4 FDD TDD UL-DLconfiguration 2 8 6 6 FDD TDD UL-DL configuration 3 8 5 4 FDD TDD UL-DLconfiguration 4 8 6 6 FDD TDD UL-DL configuration 5 8 7 7 FDD TDD UL-DLconfiguration 6 8 4 4

In Table 13, alt 2-2 denotes an M_(DL) _(_) _(HARQ) value according toMethod 2-2. For example, when the TDD UL-DL configuration of the SCellis 0, M_(DL) _(_) _(HARQ) for the corresponding SCell is 4. When the TDDUL-DL configuration of the SCell is 1, M_(DL) _(_) _(HARQ) for thecorresponding SCell is 4. When the TDD UL-DL configuration of the SCellis 3, M_(DL) _(_) _(HARQ) for the corresponding SCell is 4.

Method 2-2 may be expressed by the following construction.

TABLE 14 if M′_(DL) _(—) _(HARQ)∈ {4,6,7,8,9,10,12,15}, M_(DL) _(—)_(HARQ)=M′_(DL) _(—) _(HARQ) else M_(DL) _(—) _(HARQ) such that M′_(DL)_(—) _(HARQ) > largest value of {4,6,7,8,9,10,12,15}

Here, M′_(DL) _(_) _(HARQ) denotes a new M_(DL) _(_) _(HARQ) valuegenerated through Method 1, and M_(DL) _(_) _(HARQ) denotes a finalM_(DL) _(_) _(HARQ) value obtained based on Method 2-2.

The above described Method 2-2 is to select the largest M_(DL) _(_)_(HARQ) value from among existing values that are smaller than acorresponding new M_(DL) _(_) _(HARQ) value generated by Method 1, whichis different from the Method 2-1. Therefore, according to Method 2-2, asize of soft channel bits utilized for a single code block may begreater than Method 2-1, and even Method 1. However, a smaller M_(DL)_(_) _(HARQ) value has been taken into consideration and thus, “HARQblocking” may occur and performance of the UE may be deteriorated due toa plurality of NACKs and a plurality of times of retransmissionsincurred by a poor channel condition. However, this may not frequentlyoccur. Here, “HARQ blocking” refers to a situation in which a softbuffer that operates based on a smaller number of HARQ processes thanthe number of HARQ processes for a possible retransmission, is requiredto store a larger number of soft channel bits than the assumed number ofsoft channel bits at a predetermined point in time, and fails to store afew of the soft channel bits due to an insufficient memory.

Method 3: A Method of Storing Soft Channel Bits Preferentially for aPCell or an FDD Serving Cell

Method 3 applies a soft buffer partitioning method different for eachserving cell, unlike Method 1 and Method 2. Both the described Method 1and Method 2 have a soft buffer size identical for each serving cell,and control a soft buffer by defining and applying different M_(DL) _(_)_(HARQ) values in the soft buffer size divided for each serving cell.However, according to Method 3, a soft buffer size may be different foreach serving cell. In particular, by executing soft buffer partitioning,the present invention allocates a soft buffer size preferentially for aPCell or an FDD serving cell.

FIG. 9 illustrates an example of a soft buffer partitioning methodaccording to Method land Method 2, and a soft buffer partitioning methodaccording to Method 3.

Referring to FIG. 9, an identical soft buffer partition is applied to aPCell and an SCell according to Method 1 and Method 2, whereas differentsoft buffer partitions may be applied to a PCell and an SCell accordingto Method 3. In this instance, the number of soft channel bits for PDCSHtransmission may be configured to be large for a predetermined servingcell, for example, a PCell or an FDD serving cell. For example, when thenumber of soft channel bits for PDSCH transmission with respect to aPCell or an FDD serving cell is configured to be relatively large, thereliability of PDSCH transmission executed on the PCell or the FDDserving cell may be improved, or the reliability of PDSCH transmissionexecuted on an FDD serving cell including a larger number of DLsubframes than a TDD serving cell may be improved.

To configure the number of soft channel bits to be large with respect toa predetermined serving cell, a weight factor α for the predeterminedserving cell may be introduced. Here, a may be defined as apredetermined value, or may be transmitted from a base station to a UEthrough an RRC signaling. For example, a may be included in TDD-FDD CAconfiguration information and may be transmitted from a base station toa UE.

(1) Method 3-1

Method 3-1 is to equally allocate, to the remaining serving cells, theremaining area after excluding a soft buffer size allocated to apredetermined serving cell, by summing all min(M_(DL) _(_) _(HARQ) _(_)_(i), M_(limit)) values of the remaining serving cells.

FIG. 10 illustrates an example of soft buffer allocation according toMethod 3-1.

Referring to FIG. 10, when a PCell, an SCell 0, and an SCell 1 areconfigured for a UE, a larger number of soft channel bits are allocatedto the PCell (or an FDD serving cell), in comparison to other servingcells. For the remaining serving cells SCell 0 and the SCell 1 afterexcluding the PCell, the remaining soft buffer area may be equallyallocated.

The number of soft channel bits for a single code block in apredetermined serving cell such as the PCell (or the FDD serving cell)may be based on the following Equation 2.

$\begin{matrix}{{n_{SB} = {\min( {N_{cb},\lfloor \frac{N_{soft}^{\prime} \cdot \alpha}{C \cdot N_{cells}^{DL} \cdot K_{MIMO} \cdot {\min( {M_{DL\_ HARQ},M_{limit}} )}} \rfloor} )}},{1 \leq \alpha < N_{cells}^{DL}}} & \lbrack {{Equation}\mspace{14mu} 2} \rbrack\end{matrix}$

Here, α is a weight factor, and when α is 1, it is identical to Equation1.

The number of soft channel bits for a single code block in the remainingserving cells after excluding the predetermined serving cell may beexpressed by the following Equation 3 or 4.

$\begin{matrix}{n_{SB} = {\min( {N_{cb},\lfloor \frac{N_{soft}^{\prime}( {N_{cells}^{DL} - \alpha} )}{C \cdot N_{cells}^{DL} \cdot K_{MIMO} \cdot {\sum\limits_{i = 2}^{N_{cells}^{DL}}{\min( {M_{{DL\_ HARQ}{\_ i}},M_{limit}} )}}} \rbrack} )}} & \lbrack {{Equation}\mspace{14mu} 3} \rbrack \\{n_{SB} = {\min( {N_{cb},\lfloor \frac{N_{soft}^{\prime}( {N_{cells}^{DL} - \alpha} )}{C \cdot N_{cells}^{DL} \cdot K_{MIMO} \cdot {\sum\limits_{i = N_{cells}^{FDD}}^{N_{cells}^{DL}}{\min( {M_{{DL\_ HARQ}{\_ i}},M_{limit}} )}}} \rbrack} )}} & \lbrack {{Equation}\mspace{14mu} 4} \rbrack\end{matrix}$

Here, Equation 3 corresponds to a case in which only the PCell isweighted, and Equation 4 corresponds to a case in which only the FDDserving cell is weighted. N^(FDD) _(cells) denotes the number of FDDserving cells configured for the UE in the case of TDD-FDD CA.

(2) Method 3-2

Unlike Method 3-1, Method 3-2 is to equally divide the remaining areaafter excluding the soft buffer size allocated to a predeterminedserving cell, for the remaining serving cells, and to execute allocationof an equally divided area for each serving cell based on min(M_(DL)_(_) _(HARQ) _(_) _(i), M_(limit))

FIG. 11 illustrates an example of soft buffer allocation according toMethod 3-2.

Referring to FIG. 11, when a PCell, an SCell 0, and an SCell 1 areconfigured for a UE, a larger number of soft channel bits are allocatedto the PCell (or an FDD serving cell), in comparison to other servingcells. Although the soft buffer area sizes of the remaining servingcells SCell 0 and SCell 1 after excluding the PCell are identical, asoft buffer area may be allocated to the SCell 0 and the SCell 1 basedon a corresponding min(M_(DL) _(_) _(HARQ) _(_) _(i), M_(limit)).

The number of soft channel bits for a single code block in apredetermined serving cell such as the PCell (or the FDD serving cell)may be based on the following Equation 5.

$\begin{matrix}{{n_{SB} = {\min( {N_{cb},\lfloor \frac{N_{soft}^{\prime} \cdot \alpha}{C \cdot N_{cells}^{DL} \cdot K_{MIMO} \cdot {\min( {M_{DL\_ HARQ},M_{limit}} )}} \rfloor} )}},{1 \leq \alpha < N_{cells}^{DL}}} & \lbrack {{Equation}\mspace{14mu} 5} \rbrack\end{matrix}$

Here, α is a weight factor, and when α is 1, it is identical toEquation 1. The number of soft channel bits for a single code block inthe remaining serving cells after excluding the predetermined servingcell may be expressed by the following Equation 6 or 7.

$\begin{matrix}{n_{SB} = {\min( {N_{cb},\lfloor \frac{N_{soft}^{\prime}( {N_{cells}^{DL} - \alpha} )}{C \cdot K_{MIMO} \cdot ( {N_{cells}^{DL} - 1} ) \cdot {\min( {M_{{DL\_ HARQ}{\_ i}},M_{limit}} )}} \rfloor} )}} & \lbrack {{Equation}\mspace{14mu} 6} \rbrack \\{n_{SB} = {\min( {N_{cb},\lfloor \frac{N_{soft}^{\prime}( {N_{cells}^{DL} - \alpha} )}{C \cdot K_{MIMO} \cdot ( {N_{cells}^{DL} - N_{cells}^{FDD}} ) \cdot {\min( {M_{{DL\_ HARQ}{\_ i}},M_{limit}} )}} \rfloor} )}} & \lbrack {{Equation}\mspace{14mu} 7} \rbrack\end{matrix}$

Here, Equation 6 corresponds to a case in which only the PCell isweighted, and Equation 7 corresponds to a case in which only the FDDserving cell is weighted. N^(FDD) _(cells) denotes the number of FDDserving cells configured for the UE in the case of TDD-FDD CA.

Through Method 3 as described above, the UE may apply a weight to securereliable reception for PDSCH transmission executed on a predeterminedserving cell(s), and may control the weight. Method 3 may obtain aresult identical to the above described Method 1 or Method 2, dependingon a weight value, and may give the degree of freedom between anembodiment and a performance. In this instance, an M_(DL) _(_) _(HARQ)value used in Method 3 may use an existing value, and may use a newM_(DL) _(_) _(HARQ) value, defined based on Method 1, Method 2, orMethod 4 to be described below.

Method 4: A Method of Applying M_(DL) _(_) _(HARQ)=M_(limit) to AllServing Cells

Method 4 applies M_(DL) _(_) _(HARQ)=M_(limit) to all serving cells,irrespective of the TDD-FDD CA and a scheduling mode (that is,irrespective of Case 1 through Case 4 of Table 7). Therefore, M_(DL)_(_) _(HARQ)=M_(limit)=8 with respect to all serving cells, irrespectiveof a duplex mode. In this instance, M_(limit) is an example, and mayhave a different value depending on cases. According to Method 4,performance deterioration may occur in some serving cells and HARQblocking may be incurred depending on cases but this may be readilyembodied.

FIG. 12 illustrates an example of soft buffer allocation according toMethod 4.

Referring to FIG. 12, a soft buffer of an identical size is allocated toall serving cells that configure the TDD-FDD CA configuration for a UE,and an identical number of soft channel bits are stored.

According to Method 4, above mentioned Equation 1 may be expressed asfollows.

$\begin{matrix}{n_{SB} = {\min( {N_{cb},\lfloor \frac{N_{soft}^{\prime}}{C \cdot N_{cells}^{DL} \cdot K_{MIMO} \cdot M_{limit}} \rfloor} )}} & {{Equation}\mspace{14mu} 8}\end{matrix}$

Method 5: A Method of Executing Soft Buffer Partitioning Based on a NewM_(limit) Value

Method 5 has proposed a new M_(limit) value, instead of an existingM_(limit)=8. In this instance, soft buffer partitioning may be executedbased on the new M_(limit) value, and soft buffer partitioning may beexecuted further based on a total sum of the number of HARQ processes ofall serving cells. The reason that defines the new M_(limit) value isfor Case 2 of Table 7, and for Case 3/4 potentially.

FIG. 13 illustrates examples of the maximum number of DL HARQ processesaccording to Case 1 and Case 2.

Referring to FIG. 13, Case 1 corresponds to a case in which anFDD(PCell)-TDD(SCell) CA and self-scheduling are configured for a UE,and Case 2 corresponds to a case in which a TDD(PCell)-FDD(SCell) CA andself-scheduling are configured for a UE. In addition, alt 1 correspondsto a case that uses an existing method to count the number of DL HARQprocesses corresponding to a DL reference configuration-based HARQtiming, and alt 2 corresponds to a case that counts the number of DLHARQ processes by taking into consideration a DL subframe that isavailable for DL PDSCH scheduling.

In FIG. 13, Method 1 (alt 2) has been applied to Case 1 and Case 2, andit is recognized that all of the M_(DL) _(_) _(HARQ) values for an SCellin Case 2 are greater than an existing M_(limit)=8, unlike Case 1. A DLHARQ timing method applicable in Case 2 may include a method of applyinga new DL HARQ timing and a method of applying a DL reference timing. Inparticular, when a new DL HARQ timing is applied, a M_(DL) _(_) _(HARQ)value is significantly greater than M_(limit)=8. Therefore, when theexisting M_(limit) value is maintained as it is, a great number of timesof “overbooking” may occur in Case 2. Here, “overbooking” refers to acase in which M_(DL) _(_) _(HARQ) is greater than 8, and min(M_(DL) _(_)_(HARQ), M_(limit)) is always limited to 8.

In the existing TDD, the overbooking is allowed. For example,overbooking may occur when a TDD UL-DL configuration corresponds to TDDUL-DL configuration 2, 4, or 5. However, for ease of embodiment and forthe support of a dual mode UE (an existing UE that supports both an FDDmode and a TDD mode), the M_(limit) value is limited to 8. However, whena TDD-FDD CA has already been configured for a UE and has operated, thesupport of the dual mode UE that operates as one of a TDD carrier and anFDD carrier may become insignificant. Therefore, a new M_(limit) valuemay be required for improving performance of a UE. A relatively higherM_(limit) value may provide a low HARQ blocking probability, and arelatively lower M_(limit) value may enable a greater number of softchannel bits per TB to be stored and thus, a higher reliability may beprovided. Therefore, a HARQ process performance of the UE may becontrolled by setting an M_(limit) value based on a situation.Hereinafter, descriptions will be provided under the assumption that arelatively higher M_(limit) value is set.

(1) Method 5-1

When a new M_(limit) value greater than an existing value is allowed formin(M_(DL) _(_) _(HARQ), M_(limit)) of Equation 1, the number of DL HARQprocesses where overbooking occur may be minimized in Case 2 or thelike. Here, the new M_(limit) value may have a fixed value for apredetermined TDD-FDD CA configuration (for example Case 2), or may beindicated through an RRC signaling.

FIG. 14 illustrates an example of soft buffer allocation according toMethod 5-1. FIG. 14 corresponds to a case in which a PCell is a carrierof TDD UL-DL configuration 0, an SCell is an FDD carrier, andself-scheduling is configured for a UE. In addition, FIG. 14 assumesthat M_(limit) is 12.

Referring to FIG. 14, P-0, P-1, and the like denote DL HARQ processes ofa PCell, and S-0, S-1, and the like denote DL HARQ processes of anSCell. As illustrated in FIG. 14, M_(limit) is 12 and thus, overbookingoccurs in S-12 and S-13. In comparison with an existing case that has 6DL HARQ processes where overbooking occur when M_(limit)=8, it isrecognize that there may be a smaller number of DL HARQ processes whereoverbooking occur.

By setting an M_(limit) value to a higher value than an existing valueof 8, a fewer number of times of process drop may occur. However, when aPCell is a carrier of TDD UL-DL configuration 0 and an SCell is an FDDcarrier in Case 2, the PCell has a maximum of four DL HARQ processes andthe SCell has a maximum of 14 DL HARQ processes, which have a greatdifference between them. When a difference in the maximum number of DLHARQ processes between the serving cells is high, a large number of DLHARQ processes may be dropped, and particularly, HARQ blocking mayfrequently occur when a channel environment is poor and thus, theperformance of the UE may be deteriorated.

(2) Method 5-2

Method 5-2 is to set an M_(limit) value to be higher than an existingvalue of 8, and to add M_(DL) _(_) _(HARQ) values of all of the servingcells so as to enable a code block of each serving to have an identicalnumber of soft channel bits. This may be listed, as shown below.

$\begin{matrix}{n_{SB} = {\min( {N_{cb},\lfloor \frac{N_{soft}^{\prime}}{C \cdot K_{MIMO} \cdot {\sum\limits_{i = 0}^{N_{cells}^{DL} - 1}{\min( {M_{DL\_ HARQ},M_{newlimit}} )}}} \rfloor} )}} & \lbrack {{Equation}\mspace{14mu} 9} \rbrack\end{matrix}$

FIG. 15 illustrates an example of soft buffer allocation according toMethod 5-2. FIG. 15 corresponds to a case in which a PCell is a carrierof TDD UL-DL configuration 0, an SCell is an FDD carrier, andself-scheduling is configured for a UE. In addition, FIG. 15 assumesthat M_(limit) is 12.

Referring to FIG. 15, P-0, P-1, and the like denote DL HARQ processes ofa PCell, and S-0, S-1, and the like denote DL HARQ processes of anSCell. In FIG. 15, an identical number of soft channel bits may beallocated to each DL HARQ process of the PCell and the SCell.

Method 6: A Method of Applying an M_(DL) _(_) _(HARQ) Value of a PCellto Another Serving Cell (SCell)

Method 6 is to apply an M_(DL) _(_) _(HARQ) value of the PCell as anM_(DL) _(_) _(HARQ) value of another serving cell. In this instance, asoft buffer may be partitioned based on an M_(DL) _(_) _(HARQ) value ofthe PCell with respect to all of the serving cells, irrespective of a DLHARQ timing of the SCell. Method 6 may be applied to all of the cases,or may be applied to a predetermined case (for example, Case 2) as asub-optimal method.

FIG. 16 illustrates an example of soft buffer allocation according toMethod 6.

Referring to FIG. 16, a soft buffer of a UE may be equally allocated toa PCell and an SCell.

When at least one of Method 1 to Method 6 of the present invention isapplied, a UE for which a TDD-FDD CA is configured may effectivelycontrol a soft buffer, and may enhance the HARQ reliability.

FIG. 17 is a flowchart illustrating a soft buffer controlling method ofa UE according to an exemplary embodiment of the present invention.

A UE receives TDD-FDD CA configuration information from a base stationin operation S1700. The UE receives the TDD-FDD CA configurationinformation through an RRC signaling. In addition, the UE may furtherreceive cross-carrier scheduling configuration information from the basestation (not illustrated).

The UE configures a TDD-FDD CA based on the TDD-FDD CA configurationinformation in operation S1710. For example, based on the TDD-FDD CAconfiguration information, the UE may configure a PCell as a TDDcarrier, and may configure an SCell as an FDD carrier, in aself-scheduling mode (Case 1). As another example, based on the TDD-FDDCA configuration information, the UE may configure a PCell as an FDDcarrier, and may configure an SCell as a TDD carrier, in aself-scheduling mode (Case 2). As another example, based on thecross-carrier scheduling configuration information, the UE may configurea PCell as a TDD carrier and may configure an SCell as an FDD carrier,in a cross-carrier scheduling mode (Case 3). As another example, basedon the cross-carrier scheduling configuration information, the UE mayconfigure a PCell as an FDD carrier and may configure an SCell as a TDDcarrier, in a cross-carrier scheduling mode (Case 4).

The UE may calculate the number of soft channel bits per code block,which are stored in a soft buffer of the UE for each serving cell, basedon the TDD-FDD CA configuration information in operation S1720. Thenumber of soft channel bits per code block may be calculated based onEquation 1. In addition, the number of soft channel bits per code blockmay be calculated based on at least one of Method 1 to Method 6.

For example, for the SCell, the number of soft channel bits per codeblock may be calculated by detecting M_(DL) _(_) _(HARQ) which is themaximum number of DL HARQ processes, by taking into consideration a DLsubframe that is actually available for DL PDSCH scheduling on theSCell, and applying the same to the above mentioned Equation 1.

As another example, for the SCell, the number of soft channel bits percode block may be calculated by selectively using an existing M_(DL)_(_) _(HARQ) when a DL HARQ timing of the PCell is applied and a newM_(DL) _(_) _(HARQ) which is obtained by taking into account a DLsubframe that is actually available for DL PDSCH scheduling on theSCell.

As another example, the number of soft channel bits per code block maybe calculated based on a weight factor α for a predetermined servingcell. In this instance, the number of soft channel bits per code blockof the predetermined serving cell to which the weight factor α isapplied may be calculated based on Equation 2 or Equation 5. In thisinstance, the number of soft channel bits per code block of theremaining serving cells may be calculated based on Equation 3, 4, 6, or7, respectively.

As another example, the number of soft channel bits per code block maybe calculated by setting an M_(DL) _(_) _(HARQ) value to be identical toan M_(limit) value for all of the serving cells. In this instance, thenumber of soft channel bits per code block may be calculated based onEquation 8.

As another example, the number of soft channel bits per code block maybe calculated by defining a new M_(limit) value. For example, theM_(limit) value may be set to be lower or higher than the existing valueof 8. In addition, an identical number of soft channel bits per codeblock may be allocated to each serving cell by setting the M_(limit)value to be higher than the existing value of 8, and adding M_(DL) _(_)_(HARQ) values of all of the serving cells. In this instance, the numberof soft channel bits per code block may be calculated based on Equation9.

Any one of the above described methods may be applied to Case 1 to Case4, or any one of the methods may be applied to one of Cases 1 to 4 andanother method may be applied to another case.

The UE may execute HARQ processes associated with a plurality ofComponent Carriers (CCs), based on the number of soft channel bits percode block of each serving cell.

FIG. 18 is a block diagram illustrating a UE according to an exemplaryembodiment of the present invention.

Referring to FIG. 18, a UE 1800 includes a communication unit 1810 and aprocessor 1820. The processor 1820 executes a process and a control foroperations of the above described present invention. The processor 1820may include an RRC processing unit 1811 and a HARQ processing unit 1812.

The communication unit 1810 receives TDD-FDD CA configurationinformation from a base station. The communication unit 1810 receivesthe TDD-FDD CA configuration information through an RRC signaling. Inaddition, the communication unit 1810 may further receive cross-carrierscheduling configuration information from the base station.

The RRC processing unit 1821 applies a TDD-FDD CA configuration to theUE 1800 based on the TDD-FDD CA configuration information. In addition,the RRC processing unit 1821 may apply cross-carrier scheduling to theUE 1800 based on the cross-carrier scheduling configuration information.

The HARQ processing unit 1822 calculates the number of soft channel bitsper code block, which are stored in a soft buffer of the UE for eachserving cell, based on the TDD-FDD CA configuration information. TheHARQ processing unit 1822 may calculate the number of soft channel bitsper code block based on Equation 1. In addition, the number of softchannel bits per code block may be calculated based on at least one ofMethod 1 to Method 6.

For example, the HARQ processing unit 1822 may calculate the number ofsoft channel bits per code block by detecting M_(DL) _(_) _(HARQ) whichis the maximum number of DL HARQ processes, by taking into considerationa DL subframe that is actually available for DL PDSCH scheduling on theSCell, and applying the same to the above mentioned Equation 1, for theSCell.

As another example, the HARQ processing unit 1822 may calculate thenumber of soft channel bits per code block by selectively using anexisting M_(DL) _(_) _(HARQ) when a DL HARQ timing of the PCell isapplied and a new M_(DL) _(_) _(HARQ) which is obtained by taking intoaccount a DL subframe that is actually available for DL PDSCH schedulingon the SCell, for the SCell.

As another example, the HARQ processing unit 1822 may calculate thenumber of soft channel bits per code block based on a weight factor αfor a predetermined serving cell. In this instance, the HARQ processingunit 1822 may calculate the number of soft channel bits per code blockof the predetermined serving cell to which the weight factor α isapplied, based on Equation 2 or Equation 5. In this instance, the HARQprocessing unit 1822 may calculate the number of soft channel bits percode block for the remaining serving cells, based on Equation 3, 4, 6,or 7.

As another example, the HARQ processing unit 1822 may calculate thenumber of soft channel bits per code block by setting an M_(DL) _(_)_(HARQ) value to be identical to an M_(limit) value for all of theserving cells. In this instance, the HARQ processing unit 1822 maycalculate the number of soft channel bits per code block based onEquation 8.

As another example, the HARQ processing unit 1822 may calculate thenumber of soft channel bits per code block by defining a new M_(limit)value. For example, the M_(limit) value may be set to be lower or higherthan the existing value of 8. In addition, each serving cell may have anidentical number of soft channel bits per code block by setting theM_(limit) value to be higher than the existing value of 8, and addingM_(DL) _(_) _(HARQ) values of all of the serving cells. In thisinstance, the HARQ processing unit 1822 may calculate the number of softchannel bits per code block based on Equation 9.

The HARQ processing unit 1822 may execute HARQ processes associated witha plurality of Component Carriers (CCs) in a TDD-FDD CA environment,based on the number of soft channel bits per code block for each servingcell.

The above description is to explain the technical aspects of exemplaryembodiments of the present invention, and it will be apparent to thoseskills in the art that modifications and variations can be made withoutdeparting from the spirit and scope of the present invention. Thus, itis intended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of performing a Hybrid Automatic RepeatreQuest (HARQ) operation by a user equipment (UE), the methodcomprising: establishing a Radio Resource Control (RRC) connection witha base station through a first serving cell, the first serving cellsupporting a Time Division Duplex (TDD) mode; receiving an RRC messagefrom the base station through the first serving cell, the RRC messagecomprising carrier aggregation (CA) configuration information, the CAconfiguration information comprising information of a second servingcell supporting a Frequency Division Duplex (FDD) mode, and the firstserving cell and the second serving cell being aggregated by a TDD-FDDCA scheme; and when the second serving cell is a secondary serving cell(SCell) associated with the first serving cell through the TDD-FDD CAscheme, executing, by the UE, a process comprising: determining amaximum number of DL HARQ processes for the second serving cell based onan uplink(UL)/downlink(DL) configuration of the first serving cell,wherein the UL/DL configuration is a DL reference UL/DL configurationfor the second serving cell; and storing soft channel bits for receivedtransport blocks (TBs) based on the maximum number of DL HARQ processes,wherein the DL reference UL/DL configuration for the second serving cellis one of 0, 1, 2, 3, 4, 5, and 6, and wherein the maximum number of DLHARQ processes is 16 when the DL reference UL/DL configuration for thesecond serving cell is 5, and the maximum number of DL HARQ processes is12 when the DL reference UL/DL configuration for the second serving cellis
 6. 2. The method of claim 1, wherein the soft channel bits arecalculated by the following equation:$n_{SB} = {\min( {N_{cb},\lfloor \frac{N_{soft}^{\prime}}{C \cdot N_{cells}^{DL} \cdot K_{MIMO} \cdot {\min( {M_{DL\_ HARQ},M_{limit}} )}} \rfloor} )}$where N′_(soft) denotes a total number of soft channel bits of the UE, Cdenotes the number of code blocks forming a single TB, N_(cb) denotes asoft buffer size of a code block, K_(MIMO) is 2 when the UE correspondsto a MIMO Transport Mode™, K_(MIMO) is 1 when the UE does not correspondto a MIMO TM, M_(limit) is 8, N^(DL) _(cells) denotes the number ofserving cells configured for the UE, and M_(DL) _(_) _(HARQ) denotes themaximum number of DL HARQ processes.
 3. The method of claim 1, whereinthe maximum number of DL HARQ processes is 10 when the DL referenceUL/DL configuration for the second serving cell is 0, wherein themaximum number of DL HARQ processes is 11 when the DL reference UL/DLconfiguration for the second serving cell is 1, wherein the maximumnumber of DL HARQ processes is 12 when the DL reference UL/DLconfiguration for the second serving cell is 2, and wherein the maximumnumber of DL HARQ processes is 15 when the DL reference UL/DLconfiguration for the second serving cell is 3, and wherein the maximumnumber of DL HARQ processes is 16 when DL reference UL/DL configurationfor the second serving cell is
 4. 4. The method of claim 1, wherein anumber of the TBs is determined based on a minimum value between themaximum number of DL HARQ processes and a reference value.
 5. The methodof claim 4, wherein the reference value is more than
 8. 6. A method ofperforming a Hybrid Automatic Repeat reQuest (HARQ) operation by a userequipment (UE), the method comprising: establishing a Radio ResourceControl (RRC) connection with a base station through a first servingcell or a second serving cell, the second serving cell supporting aFrequency Division Duplex (FDD) mode, the first serving cell supportinga Time Division Duplex (TDD) mode; receiving an RRC message from thebase station through the first serving cell or the second serving cell,the RRC message comprising carrier aggregation (CA) configurationinformation, the CA configuration information comprising information ofthe first serving cell and the second serving cell being aggregated by aTDD-FDD CA scheme; and when the first serving cell is a secondaryserving cell (SCell) associated with the second serving cell through theTDD-FDD CA scheme, executing, by the UE, a process comprising: setting,based on the determined FDD-TDD CA scheme, a first maximum number of DLHARQ processes for the first serving cell as 8; and storing soft channelbits for received transport blocks (TBs) based on the first maximumnumber of DL HARQ processes for the first serving cell set as 8; andwhen the second serving cell is a secondary serving cell (SCell)associated with the first serving cell through the TDD-FDD CA scheme,executing, by the UE, a process comprising: determining a second maximumnumber of DL HARQ processes for the second serving cell based on anuplink(UL)/downlink(DL) configuration of the first serving cell, whereinthe UL/DL configuration is a DL reference UL/DL configuration for thesecond serving cell; and storing soft channel bits for receivedtransport blocks (TBs) based on the second maximum number of DL HARQprocesses, wherein the DL reference UL/DL configuration for the secondserving cell is one of 0, 1, 2, 3, 4, 5, and 6, and wherein the secondmaximum number of DL HARQ processes is 16 when the DL reference UL/DLconfiguration for the second serving cell is 5, and the second maximumnumber of DL HARQ processes is 12 when the DL reference UL/DLconfiguration for the second serving cell is
 6. 7. The method of claim6, wherein the TDD-FDD CA scheme configures the first serving cell as aPrimary serving cell (PCell), configures the second serving cell as thesecondary serving cell (SCell), and configures a self-scheduling for theUE.
 8. A method of performing a Hybrid Automatic Repeat reQuest (HARQ)operation by a user equipment (UE), the method comprising: establishinga Radio Resource Control (RRC) connection with a base station through afirst serving cell or a second serving cell, the second serving cellsupporting a Frequency Division Duplex (FDD) mode, the first servingcell supporting a Time Division Duplex (TDD) mode; receiving an RRCmessage from the base station through the first serving cell or thesecond serving cell, the RRC message comprising carrier aggregation (CA)configuration information, the CA configuration information comprisinginformation of the first serving cell and the second serving cell beingaggregated by a TDD-FDD CA scheme; and when the second serving cell is asecondary serving cell (SCell) associated with the first serving cellthrough the TDD-FDD CA scheme, executing, by the UE, a processcomprising: determining a maximum number of DL HARQ processes for thesecond serving cell based on an uplink(UL)/downlink(DL) configuration ofthe first serving cell, wherein the UL/DL configuration is a DLreference UL/DL configuration for the second serving cell, wherein themaximum number of DL HARQ processes is determined based on the followingTable TABLE DL-reference Maximum number UL/DL Configuration of HARQprocesses 0 10  1 11  2 12  3 15  4 16  5 16  6
 12.


9. The method of claim 8, the process further comprising: storing softchannel bits for received transport blocks (TBs) based on the maximumnumber of DL HARQ processes.
 10. The method of claim 9, wherein a numberof the TBs is determined based on a minimum value between the maximumnumber of DL HARQ processes and a reference value, wherein the referencevalue is more than 8.